Baluns are used to connect an unbalanced (asymmetrical) circuit to a balanced (symmetrical) circuit. A balun can e.g. be used between a coaxial cable, being an unbalanced circuit, and a ribbon cable being a balanced circuit. Baluns are widely used in e.g. radar systems and communication systems such as wireless communication systems.
An input balun having a single, unbalanced input signal ideally produces two output signals with equal magnitude but with a phase difference of 180°. An output balun combines two input signals with a 180° phase difference to produce a single output signal.
A wire-wound transformer is a simple type of a passive balun. The unbalanced connection is made at one winding and the balanced connection at a second winding. These kinds of baluns are relative expensive, produce losses and are narrow banded.
Another passive type of balun is the LC-lumped balun shown in FIG. 1. This is the first order lattice balun very commonly used but it is very limited in bandwidth and will have an insertion loss exceeding more than 3 dB. The lumped balun has an input terminal 101, output terminals 102 and 103 and a ground connection 104. The input terminal is connected to the output terminal 102 through a first capacitor 105 and to output terminal 103 through first inductor 108. This means that there will be a phase difference between the output terminals of 180° and by choosing suitable values of the first capacitor 105 and the first inductor 108 the signals at the output terminals 102 and 103 will have the same amplitude. The output terminal 103 will be connected to the ground through a second capacitor 106 and the output terminal 102 will be connected to the ground 104 through a second inductor 107. This has the effect that the two output terminals will be balanced in relation to the ground.
There are also active baluns involving active components like transistors. One example of such an active balun is described in U.S. Pat. No. 4,994,755. FIG. 2 shows the principle of this active input balun 200. DC biasing components, DC blocking capacitors and output transmission line sections are not shown in the figure for clarity reasons. An input transmission line 201 comprises transmission line sections 201a-201c which are successively coupled to inputs 202a-202c of amplifiers 203a-203c. The amplifiers can e.g. comprise FET transistors. The outputs, 204a-204c, of the amplifiers 203a-203c are also successively coupled to an output transmission line 205 comprising series coupled transmission line sections 206a-206b via transmission line sections 207a-207c as shown in FIG. 2. An RF (Radio Frequency) signal is fed at input terminal 208 and propagates through the input transmission line 201, with portions of said signal being coupled to the amplifiers 203a-203c. At the end of the input transmission line 201 the remaining portion of the RF signal is coupled to a first output terminal 209. The gain provided by each of the amplifiers to the signal portions coupled through them are selected such that at a second output terminal 210, the signal portions through the amplifiers are added in phase with the 180° phase shift provided by the transistor elements in the amplifiers 203. The gain of the amplifiers are adjusted such that the amplitude of the output signal at the second terminal 210 is substantially the same as at the first output terminal 209. The gain of the amplifiers can be adjusted to compensate for some of the transmission losses. However the RF output level at the first output terminal 209 must be lower then the RF amplitude at the input terminal 208 as there is no gain in the input transmission line. In order to get amplitude balance at the output terminals the amplitude of the RF signal at the second output terminal 210 also has to be below the amplitude level at the input terminal. By using the distributed/travelling wave principle where the input RF-signal is successively propagated to a second transmission line through a distributed capacitance this solution has a relatively broad bandwidth. A severe drawback is however that the amplitude level of each balanced RF output signal, henceforth called the RF-out signal, has to be below the amplitude level of the RF input signal, henceforth called the RF-in signal, in order to obtain amplitude balance between the output signals.
Thus there is a need to provide a balun having an improved bandwidth in combination with providing RF-out signals with good amplitude balance and a mutual phase difference of 180° and with amplitude levels at the output terminals equal to or exceeding the signal level at the input terminal.